Cellular radiotelephone systems are known. Such systems are typically divided over geographical areas into substantially contiguous service coverage areas, each served from a base site. Mobile communication units passing through a service coverage area of a base site typically transmit service requests to the base site, which request is answered by the base site with a resource allocation for use by the requesting communication unit. Such a system is typically referred to as a frequency division multiple access system (FDMA).
Communication resources used by a base site, and allocated to the communication system, may consist of a frequency spectrum divided for use by the system into a number of communication resources. Each communication resource may consist of a pair of frequencies. The pair of frequencies may be used by a communication unit to transmit and receive information.
Communication resources use by a base site may also be divided by function with at least one resource, in some systems, reserved for the two-way transmission of control information between the base site and communication unit. Other resources may be reserved for use for communication transactions by communication units.
Since a limited number of communication resources may exist within a frequency spectrum the reuse of communication resources is necessary within the geographic area served by the communication system. Reuse, on the other hand, is often limited by mutual interference between communication units and base sites, operating on the same frequency in the same system.
Reuse of a communication resource within a communication system is limited by a number of factors. Chief among the factors, specifically in a communication system having an omnidirectional antenna structure, is distance between reusing transmitters and the power of the transmitted signal. The distance between reusing communication units is often specified in terms of the ratio of the distance (D) between the centers of nearest, neighboring co-channel cells to the radius (r) of a cell. The reuse ratio, D/r, (reuse ratio) specifies the proximity of the closest reusing base site.
As the power of the transmitted signal is reduced the proximity between reusing base sites may also be reduced. One patent teaching of such an approach is that of Cunningham et al. (U.S. Pat. No. 4,144,496). Cunningham teaches of a method of subdividing a large cell into a number of smaller cells. Communication resources assigned to the large cell are divided among the smaller cells. Directional antennas are also used in Cunningham to isolate reusing base sites in an effort to reduce co-channel interference.
The use of directional antennas was also taught in Graziano (U.S. Pat. No. 4,128,740) as a method of reducing the reuse factor. Directional antennas were used in Graziano in combination with a resource assignment algorithm to produce a co-channel reuse ratio of 4.
Advances (described in Cunningham) resulting in a reduced reuse factor, in the prior art, have been drawn to the development of directional antennas and selective assignment of frequencies. Past developments (as in Graziano) have included cells divided into as many as six sectors with a 60 degree sector antenna providing coverage within each sector. The selective assignment of resources then precluded reassignment of adjacent channel resources within adjacent sectors. Use of the 60 degree sector antennas, and the selective assignment of frequencies, as mentioned, have resulted in reuse factors as low as four.
Other approaches, such as described in Lee (U.S. Pat. No. 4,932,049), have been to locate a number of directional antenna sets around the periphery of a service coverage area, directed into the cell, and to provide service to mobile communication units from the nearest antenna. Included within the Lee system is a master antenna set and a number of slave antenna sets. Control information, within the Lee system, is exchanged with mobiles from a directional antenna located at the master site to mobiles throughout the service area.
While past efforts have been effective in reducing reuse distances, difficulties are often experienced. Under Lee, Cunningham, Graziano, or other prior art techniques, positioning of base sites and directionality of antennas becomes critical for effective cell coverage. Where cells have been divided, the division of frequencies among the divided cells further complicates the ability of a system to accommodate dynamic loading fluctuations within the system. Natural and man-made obstructions, such as hills or tall buildings, often interfere with transmissions within such systems.
While the transmission difficulties offered by hills or tall buildings may be overcome by multiple antenna systems, as in Lee, the difficulties offered by directional antennas remain. Further, because the Lee control antenna must transceive from the periphery of the cell, across the full diameter of the cell, the reduced-power advantage of the Lee multiple antenna cell is partially negated by the power that must be used in transmitting control signals.
As the number of subscribers to cellular systems increases the size of cells must be reduced to accommodate the heavier loads. Such loads are accommodated through the reduced reuse distances associated with smaller cells.
As the service coverage areas of cells decrease, the problems associated with dynamic loading, and with obstructions, increase. Where service coverage areas decrease to areas of less than a mile, signal obstructions offered by buildings or tunnels become significant. Because of the need for reliable communication a need exists for a method of expanding cell capacity that is less dependent on positioning and directionality of antenna.